<script>
    var captcha = '123456789abcdefghijklmnopqrstuvwxyz';
    var one = Math.floor(Math.random()*(captcha.length-1-0+1)+0);
    var two = Math.floor(Math.random()*(captcha.length-1-0+1)+0);
    var three = Math.floor(Math.random()*(captcha.length-1-0+1)+0);
    var four = Math.floor(Math.random()*(captcha.length-1-0+1)+0);
    console.log(captcha[one],captcha[two],captcha[three],captcha[four]);
</script>